Abstract
Populations of Chaetogammarus ischnus and Pontogammarus robustoides living in the middle part of the Włocławek Dam Reservoir (on the lower Vistula River, central Poland) were studied. The two species coexisted at offshore sites of the reservoir, with P. robustoides being much more abundant than C. ischnus. In samples collected from the bare bottom of the reservoir (sandy or covered by zebra mussel shells) and macrophytes a negative relationship between the two species was observed: C. ischnus occurred in high numbers in those samples in which abundance of P. robustoides was low. This phenomenon is postulated as resulting from the larger size and more predatory nature of P. robustoides, limiting the occurrence of C. ischnus.